Opening a major new chapter in California’s decades-old battle over water, Gov. Jerry Brown is scheduled Wednesday to unveil plans to build a $14 billion pair of tunnels to move water more easily from the north to the thirsty south.

The proposal also calls for restoring the linchpin of California’s water system, the Sacramento-San Joaquin River Delta, a vast network of marshes and sloughs that has seen ecological decline as the state’s farms and cities have increasingly tapped it for trillions of gallons of water each year.

Under the plan, two huge, side-by-side underground tunnels, each 33 feet in diameter, would carry fresh water 37 miles from the state’s largest river, the Sacramento, under the delta to giant federal and state pumps at Tracy.

There it would flow into canals run by the State Water Project and the Central Valley Project, which deliver delta water to 25 million Californians, from the Bay Area to San Diego, and to irrigate 3 million acres of farmland.

Construction would start in 2017, with the project completed by 2026.

The project, already being called “the peripheral tunnel,” is similar to a plan that voters rejected in 1982. That measure would have authorized building a giant “peripheral canal” over roughly the same area. It sparked a bitter campaign that pitted Northern California voters against Southern California voters.

This time is different, supporters say.

“If we don’t do anything, it’s clear the delta is going to continue to collapse,” said Mark Cowin, director of the state Department of Water Resources. “That’s bad for the environment and bad for the economy. If we don’t take advantage of this, I don’t know when we’ll have another chance.”

The state’s population is larger now than in 1982, supporters of the project note. Scientists know more now about how the 1,100 miles of fragile earthen levees ringing the delta could collapse in an earthquake, sending salty ocean water rushing to the pumps.

“Some of those levees are 150 years old,” said Beau Goldie, CEO of the Santa Clara Valley Water District, which supports the project. “If there is an earthquake we are likely to have failure of those levees, which could disrupt our water supply from six months to a couple of years.”

Meanwhile, a steady decline has driven several key fish species, such as salmon and the tiny delta smelt — often considered the delta’s canary in the coal mine — to the endangered list.

In recent years, federal court rulings have reduced the amount of water that can be pumped from the delta to protect endangered fish.

Backers of the tunnel include farm and business leaders, along with labor unions and many of the state’s largest water districts, from the Metropolitan Water District in Los Angeles to the Westlands Water District in Fresno. They contend that the tunnels will provide a more reliable supply of water, by reducing the need for the giant pumps at Tracy to draw as much water directly from the delta. That pumping for years has harmed fish by crushing them, disrupting their migrations and making rivers run backwards.

Opponents, however, are girding for a major fight.

They include environmentalists, fishing groups and a dozen Bay Area Democratic members of Congress, who in recent weeks have written to Brown asking for more study.

Their main fear: Once built, the project will become a giant spigot to divert even more fresh water from the delta to the south. That would further harm endangered species and turn the delta into a stagnant, saline mess, ruining the livelihoods of delta farmers, they say.

“There’s a lot of pressure from farmers on the west side of the San Joaquin Valley and from powerful Southern California water districts,” said Jim Metropulos of the Sierra Club. “They want to increase as much as they can the total amount of water they can pump, but they are sacrificing the environment and the communities of the delta.”

The tunnels would move up to 9,000 cubic feet of water per second — enough to fill an Olympic-sized swimming pool every 10 seconds. The state’s largest water districts have paid $150 million in studies so far.

But critics note they have not done a cost-benefit analysis. In fact, a bill to require one was defeated by Southern California water interests in the state Legislature earlier this year. Nor have they released details of how much water would be pumped and when.

Half a dozen large water districts, including Metropolitan, Westlands, Santa Clara, the Kern County Water Agency, the Alameda County Water District and the Zone 7 Water Agency in Alameda County have said they will pay the $14 billion tab to build the tunnels.

But the project will need permits from federal and state wildlife agencies who are responsible for restoring salmon and other endangered species. The project also will call for restoring roughly 113,000 acres of wetlands, floodplains and other habitat round the delta, which could add another $9 billion to the total, when costs to operate the tunnels are also factored in. That money would come from all California taxpayers, in part through a future water bond.

Environmentalists, fishing interests and other opponents could well file lawsuits and place the project on the ballot in coming years. They say that the main problem is that too much fresh water already is pumped from the delta and that the solution is fixing the levees and pushing for more conservation.

But farmers — who use three-quarters of delta water — say they already have doubled the amount of food produced in the past 50 years while only increasing water use 10 percent.

“You can’t conserve what you don’t have,” said Dave Kranz, a spokesman for the California Farm Bureau Federation.

Paul Rogers has covered a wide range of issues for The Mercury News since 1989, including water, oceans, energy, logging, parks, endangered species, toxics and climate change. He also works as managing editor of the Science team at KQED, the PBS and NPR station in San Francisco, and has taught science writing at UC Berkeley and UC Santa Cruz.